1. Field of the Invention
The present invention relates to radiation sources, particularly discharge plasma sources emitting EUV radiation such as may be used as the radiation source of a lithographic projection apparatus comprising:
a radiation source constructed and arranged to generate extreme ultraviolet radiation;
an illumination system constructed and arranged to receive said extreme ultraviolet radiation and to supply a projection beam of said extreme ultraviolet radiation;
patterning means constructed and arranged to pattern the projection beam according to a desired pattern;
a substrate table constructed to hold a substrate; and
a projection system constructed and arranged to image the patterned beam onto target portions of the substrate.
2. Description of the Related Art
The term xe2x80x9cpatterning meansxe2x80x9d should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d has also been used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:
A mask table for holding a mask. The concept of a mask is well known in lithography, and its includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. The mask table ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference; and
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask table and mask; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as hereabove set forth.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics and catadioptric systems, for example. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target area (comprising one or more dies) on a substrate (silicon wafer) which has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target areas which are successively irradiated via the mask, one at a time. In one type of lithographic projection apparatus, each target area is irradiated by exposing the entire mask a pattern onto the target area at once such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, which is commonly referred to as a step-and-scan apparatus, each target area is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
In general, apparatus of this type contained a single mask (first object) table and a single substrate (second object) table. However, machines are becoming available in which there are at least two independently moveable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such a multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial metrology steps on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughout, which in turn improves the cost of ownership of the machine.
In a lithographic apparatus the size of features that can be imaged onto the substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. Whilst most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray and possible sources include, for instance, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings. An outline design of a lithographic projection apparatus using synchrotron radiation is described in xe2x80x9cSynchrotron radiation sources and condensers for projection x-ray lithographyxe2x80x9d, JB Murphy et al, Applied Optics Vol. 32 No. 24 pp. 6920-6929 (1993). Apparatus using discharge plasma sources are described in: W. Partlo, I. Fomenkov, R. Oliver, D. Birx, xe2x80x9cDevelopment of an EUV (13.5 nm) Light Source Employing a Dense Plasma Focus in Lithium Vaporxe2x80x9d, Proc SPIE 3997, pp. 136-156, 2000; M. W. McGeoch, xe2x80x9cPower Scaling of a Z-pinch Extreme Ultraviolet Sourcexe2x80x9d, Proc SPIE 3997, pp. 861-866, 2000; and W. T. Silfvast, M. Klosner, G. Shimkaveg, H. Bender, G. Kubiak, N. Fornaciari, xe2x80x9cHigh-power plasma discharge source at 13.5 and 11.4 nm for EUV lithographyxe2x80x9d, Proc SPIE 3676, pp. 272-275, 1999.
In a discharge plasma source, a partially ionized, low-density and relatively cold plasma is formed by an electrical discharge and then compressed so that it becomes highly ionized and reaches a very high temperature, causing the emission of EUV radiation. Preionization by, for instance, a source of RF power may be employed to start the discharge and to possibly create a well-defined plasma sheet. The geometry of devices, such as plasma focus, Z-pinch and capillary sources, may vary, but in each of these types a magnetic field generated by the electrical current of the discharge drives the compression. It is essential to optimize the efficiency and intensity of a discharge plasma source because there are few gases which have appropriate magneto-hydrodynamic properties to form a plasma which can be sufficiently compressed and which also emit a sufficiently large amount of radiation in the desired frequency band.
It is an object of the present invention to provide an improved plasma source that may be used in a lithographic projection apparatus.
According to the present invention there is provided a plasma radiation source for extreme ultraviolet electromagnetic radiation comprising:
electrodes connected to a source of high electrical potential and constructed and arranged to allow a first plasma state to compress into a pinch volume by an electrical current induced in said first plasma state and a corresponding magnetic field;
a supply for a working fluid to be brought into a high-temperature plasma state to emit extreme ultraviolet electromagnetic radiation; and
a primary jet nozzle constructed and arranged to eject said working fluid into said pinch volume so as to be brought into said high-temperature plasma state by compression of said first plasma state into said pinch volume.
Because the EUV radiation is principally emitted by the working (primary) fluid, which is raised to a high-temperature, radiation-emitting state by the compressing discharge, the working fluid can be chosen for its efficiency in emitting EUV radiation at the desired wavelength, without being limited by the need for properties favorable to forming a discharge. The working fluid may be, for example, Li vapor, Krypton, Xenon, water and cryogenic liquids. At the same time, a driver fluid, chosen on the basis of its magneto-hydrodynamic properties to be effective at generating a conducting and effective compression medium, and on the basis of its EUV optical properties, particularly its transmissivity at the wavelength(s) of interest, can be supplied to the space between the electrodes to assist in formation of the discharge. The plasma generating requirements and the emission requirements are therefore decoupled, allowing a wider choice of substance for each component and enabling improvements in the effectiveness and efficiency of the source.
The provision of fresh working fluid for each discharge (xe2x80x9cshotxe2x80x9d) of the source also increases the possible repetition rate of the source by allowing the initial state to be reached more quickly each cycle. The fresh supply of working fluid also serves to flush pollution from the pinch volume, which takes time in the prior art sources. Further more, a greater amount of the working fluid may be supplied for each discharge since it can be supplied in a more dense form, such as a cluster or liquid jet.
The density of the working fluid on the emitting axis can be increased by suitable arrangement of the primary jet so that the ejected fluid has its highest density on the emitting axis. A supersonic jet is particularly preferred as it provides a jet with a sharply peaked density profile.
In a preferred embodiment of the invention, the radiation source may further comprise:
a supply for a secondary fluid; and
a secondary jet nozzle constructed and arranged to eject said secondary fluid parallel to and spaced from the line of ejection of said working fluid.
By having a secondary jet nozzle beside the primary jet nozzle the degree of divergence of the primary gas may be decreased by the outflow of the secondary gas from the secondary jet nozzle. Since a sufficient density in the primary gas will then be present at a larger distance from the jet nozzle outlet, the plasma can thus be created at the larger distance from the nozzle outlet. This prevents the production of debris and its associated problems. Further, the radiation source may be positioned such that the outflow of secondary gas will function as a shield, for instance, between optical elements of the illumination system and parts, such as electrodes and insulators, of the source on the one hand and the high-temperature plasma created on the other hand. Such a shield will largely prevent the escape of debris particulates towards source parts or optical elements. The particulates do not pass the screening secondary gas or are slowed down and neutralised and are prevented from causing damaging effects by deposition or otherwise. Also, a radiation source having a reduced re-absorption of emitted radiation and an increased brightness may be obtained by providing for an XUV radiation transparent volume at sides of the pinch volume.
In an especially preferred embodiment, the secondary jet nozzle encloses the primary jet nozzle. In such a case, the secondary nozzle can be annular in shape or comprise a plurality of nozzles arranged to surround the primary nozzle. Such a configuration will yield an even better control over the divergence of the primary gas, and an outflow of the primary gas which is parallel or even convergent over a certain distance from the nozzle outlet can be obtained. The secondary gas enclosing the primary gas and a high-temperature plasma created therein also further prevents the escape of plasma particulates from the plasma and provides for an XUV radiation transparent volume around the pinch volume. In an optimal configuration, the primary and secondary jet nozzles are co-axial.
It should be noted that the secondary nozzle may provide an initial quantity of the secondary gas to assist the initial discharge formation and may then cease to supply gas. Alternatively the secondary gas can be supplied continuously or in pulses to perform the functions described above.
The secondary gas may comprise at least one gas selected from the group comprising helium, neon, argon, krypton, methane, silane and hydrogen or, in general, any EUV transparent gas. Hydrogen is a preferred secondary gas since it has superior absorption characteristics with respect to EUV radiation. It may thus be used in a large flow rate (high local density in the outflow), yielding a very efficient confinement of the primary gas for divergence control and screening of the plasma.
The present invention also provides a lithographic projection apparatus for imaging of a mask pattern in a mask onto a substrate, said apparatus comprising:
a radiation source constructed and arranged to generate extreme ultraviolet radiation;
an illumination system constructed and arranged to receive said extreme ultraviolet radiation and to supply a projection beam of said extreme ultraviolet radiation;
patterning means constructed and arranged to pattern the projection beam of radiation according to a desired pattern;
a substrate table constructed to hold a substrate; and
a projection system constructed and arranged to image the patterned beam onto target portions of the substrate; characterized in that:
said radiation source is as described above.
The present invention further provides device manufacturing method using a lithography apparatus comprising:
a radiation source constructed and arranged to generate extreme ultraviolet radiation;
an illumination system constructed and arranged to receive said extreme ultraviolet radiation and supply a projection beam of said extreme ultraviolet radiation;
patterning means constructed and arranged to pattern the projection beam of radiation according to a desired pattern;
a substrate table constructed to hold a substrate; and
a projection system constructed and arranged to image the patterned beam onto target portions of the substrate; the method comprising the steps of:
providing a projection beam of radiation using said radiation source;
providing a substrate that is at least partially covered by a layer of radiation-sensitive material to said substrate table;
patterning the projection beam in its cross-section according to a desired pattern;
imaging the patterned beam onto said target portions of said substrate; characterized by the step of:
using a radiation source as described above as said radiation source.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d or xe2x80x9cexposure areaxe2x80x9d, respectively.